DESCRIPTION (adapted from the applicant's description): Homeotic genes are expressed in restricted domains along the body axis of the fruitfly, Drosophila melanogaster. Domains of repression are initiated by early repressors and are maintained by a group of chromatin-associated proteins called the Polycomb group (PcG) proteins, which act through Polycomb Response Elements (PRE) or silencers. Propagation of the silent transcriptional state during development is epigenetic because only the initially repressed domains remain silenced, despite the ubiquitous presence of PcG proteins. It is not known how silencing is maintained through cell divisions, nor is it known how PcG proteins silence gene expression. These questions will be addressed by studying silencing of a homeotic gene promoter by the silencer element MCP from the Abd-B gene in the bithorax complex. The first three aims are directed at determining how the PcG silencing complex forms at the MCP silencer element, focusing on the function of Pleiohomeotic, a key PRE-binding protein required for silencing, and the mechanisms for initially establishing permanent silencing. The last two aims seek to address how the PcG complex silences gene expression, examining two potential mechanisms- chromosomal context, and chromatin structure.